Have you checked out the Limit Testing and Limit Specifications vi's in the Waveform Monitoring Pallet. There are also a couple of examples you can get to from the Labview Help - Limit Testing Measurement.vi and Limit Testing for Uneavenly Sampled Data Measurement.vi
Looks like you will have to convert your XY graph into waveform or array data to do the limit testing. Or you can take the data with negative x values and flip it to the positive side. Do the limit testing, then flip the results back to the negative side.
